19 Sep 2013

All mammals are born with a sucking reflex - an instinct on which
their lives depend - but human babies are unique in that they also need to suck
for comfort. Or so it was thought. A new study now shows evidence
suggesting that baby zebras can suckle for psychological needs, rather than
just for feeding.

Grévy's zebra foal

The use of soothers is a sensitive
topic amongst parents. Soothers (also known as pacifiers or dummies) comfort babies
and help them sleeping, but many parents go through great lengths (and many sleepless
nights) to avoid using a soother. These concerned parents may fear their baby
will develop crooked teeth, or have problems breastfeeding, or they may simply
find soothers unnatural. Regardless of their individual choices, all parents will agree that their baby has a need to suck-
whether it’s a soother, a thumb or an old rag.
There are
many studies showing that so-called 'non-nutritive sucking' cancomfort babies, help them to settle,
reduce the risk of sudden infant death syndrome (SIDS) and even increase
tolerance to pain. The use of soothers is therefore often recommended in intensive care units for premature babies and sick newborns who sadly may need painful medical procedures. But is ‘comfort sucking’ widespread amongst mammals or a
specific evolutionary adaptation of our species?

Scientists previously assumed that the
duration and frequency of suckling reflected the energetic needs of the young –
the longer an infant spent suckling, the more milk it drank. But studies in mammals directly
measuring infant weight gain and time spent suckling have shown no correlation
between the two. So why do babies and mums across
so many species invest so much time and energy nursing? One of the reasons is
bonding, but there is more to it.

Grévy's zebra mare with her foals

In a new study published in the September issue of the Journal of Zoology, a research team from the
Institute of Animal Science and Czech University of Life Sciences, in Prague, Czech
Republic, compared suckling behaviour in three zebra species - mountain, plains and Grévy's zebras. These species are
closely related but have very different social organisations, so the
researchers could ask whether time spent suckling might reflect the social needs of the young.

Mountain and plains zebras live in stable
groups, or ‘harems’, of several females, their babies (or foals) and only one
male, while Grévy’s zebras prefer to graze on their own and form loose social
bonds. Zebras are far from being docile creatures - to defend their position in
the harem social hierarchy, mountain and plains zebra females (called mares) take
their gloves off and become very aggressive. Mountain zebra mares are
especially hostile, and can sometimes even harass unrelated foals. Grévy’s
zebras are the least aggressive of the three species, perhaps because of their
more solitary nature.

The researchers observed the suckling
behaviour of 30 foals of mountain, plains and Grévy’s zebras at the Dvůr
Králové Zoo throughout several years. After watching the zebra herds for an
impressive total of about 1,500 hours, the results were clear: mountain zebra foals
suckled for longer and more frequently, followed by plains and Grévy’s zebras.
As mountain zebra herds have the highest aggression rates and Grévy’s zebra the
lowest, the team concluded that baby zebras spend more time suckling in species
where there is higher social tension.

Plains zebra foal suckling (Credit: by Chadica/Flickr)

Because the study was
performed on zebras held in captivity - and so all zebras were exposed to the same
living conditions - these differences in sucking behaviour can’t be explained
by water or food availability, or by a specific adaptation of each species to its unique
environment. But the authors of the study are nevertheless cautious about over-interpreting
their results:

“I don’t think that
the foal initiated suckling to seek comfort and stress reduction. What I think
is that the suckling bout duration reflects the psychological needs of the foal
rather than the milk transfer. Thus, prolonged suckling can reflect social
tension”, says Jan Pluháček, an ethologist at the Institute of Animal Science
and leading author in the study.

Previous studies in primates and rodents had
shown a link between maternal care and social organisation, but Pluháček and
colleagues provide new evidence suggesting that, at least in zebras, suckling
may not only be a means for the young to feed or to bond with mum- it could
also reflect their psychological needs.

“We suppose that when
any tension in the herd occurs then the young try to stay for longer with the
mother, and longer suckling can strengthen the bond between young and mother in
all mammalian species” says Pluháček “[…] in zebras 98% of suckling is
initiated by the foal. So it’s the foal who seeks soothing via suckling.”

Reference:Pluháček J. et al. (2013). Time spent suckling is affected by different social organization in three zebra species, Journal of Zoology, DOI: 10.1111/jzo.12077